add virtual destructor for SkExample to fix build error.

add virtual destructor for SkExample to fix build error.

The original SkExample have virtual function draw(), but have no virtual destructor. This may lead to memory leak when free the instance of derived class of SkExample. The latest skia code show error when build SkiaExamples by "ninja -C out/Release SkiaExamples":
error: ‘class SkExample’ has virtual functions and accessible non-virtual destructor.

R=caryclark@google.com

Author: yunchao.he@intel.com

Review URL: https://codereview.chromium.org/98463013

git-svn-id: http://skia.googlecode.com/svn/trunk@12728 2bbb7eff-a529-9590-31e7-b0007b416f81